1. Field of the Invention
The present invention relates to an electronic appliance and a power control apparatus favorably applied to controlling voltage or current in an electronic appliance.
2. Description of the Related Art
In recent years, electronic technology has come to be appreciated for its convenience and efficiency, and the spread of electronic appliances such as IT (information technology) and AV (audio visual) devices has accelerated on a global scale. At the same time, there has been increasing attention on the limited nature of the global environment and global resources, which has led to strong demand for low-energy technology in appliances.
For example, continuous improvements have been made in the efficiency of power supplies of electronic appliances, with switching power supplies, for example, achieving efficiencies of 90% and above. However, in reality, due to cost and noise considerations, many low-efficiency power supplies are still being used as before.
High-efficiency power supplies are also affected by voltage variations in the inputted power, fluctuations between components, and changes in the load current, and a large drop in efficiency is caused during low power operation, for example.
Power supplies are normally designed so as to be highly efficient at the rated load (power) of the appliance, but the operating power of an actual appliance will vary, and the efficiency of the power supply will also vary. For the example of a television set, the operating power greatly varies according to the audio output and brightness of the screen. Putting this another way, there will be an input voltage that is optimal for the size of the load current.
Since appliances are also affected by variations in the voltage of a commercial power supply, the power supply efficiency during actual operation will be lower than the appliance specification. This will be the same regardless of whether a switching regulator or a series regulator is used to supply power.
For example, although a no-load loss occurs for a typical transformer even when there is no load, resulting in minimum efficiency in the no-load state, as the load current increases, the power supply efficiency will also increase. However, since load loss is the square of the load current, once the current exceeds a certain range, the load loss will become a principal cause of the overall loss, resulting in a fall in efficiency. This relationship is shown in FIG. 1.
As one example of an actual transformerless power supply, as shown in FIG. 2, one terminal of a 100V AC commercial power supply 141 is connected via a capacitor 142 to one input terminal of a rectifier circuit 143 composed of a diode bridge, the other terminal of the commercial power supply 141 is connected to the other input terminal of the rectifier circuit 143, and a Zener diode 145 for keeping the voltage constant and a smoothing capacitor 146 are connected in parallel between the output terminals 144a and 144b of the rectifier circuit 143.
Accordingly, as shown in FIG. 2, in a transformerless power supply, by directly rectifying the commercial power supply 141 and passing the output through the Zener diode 145 that constructs a regulator, a stable DC voltage is obtained across the output terminals 144a and 144b. 
In this construction, the load of the Zener diode 145 that constructs the regulator is reduced by lowering the voltage in advance using the capacitor 142.
Capacitors are often used in low-power applications. This is because that when the voltage is dropped due to a capacitor, the current shifts out of phase with the voltage, thereby preventing a power loss from occurring. As one example, such construction is used for a standby power supply or the like. However, since the rectified output of such circuits will vary due to load variations and the like, circuits are normally constructed in accordance with the maximum load and a stabilized voltage is produced by causing a power loss at the regulator when the load is light.
Also, since the voltage drop across both ends of a capacitor greatly changes due to variations in frequency and load current, capacitors may not be used in appliances where the load current is large and load variations are large. Accordingly, at present the use of capacitors is limited to extremely low power applications of around several tens of mW, such as standby power.
Also, with the transformerless power supply shown in FIG. 2, during operation using a relay or the like where power consumption is high, by connecting the capacitor 142 in parallel with another predetermined capacitor, it is possible to increase the supplied power, but it becomes necessary to switch between a plurality of capacitors to cover a wide load range. Although switching between a plurality of capacitors using a relay or the like is theoretically possible, aside from space and cost considerations, there are also the problems of slow response, the generation of noise during switching, the inability to continuously change the capacity, and reduced durability, which make such construction impractical. Accordingly, a device where the capacitance can be continuously changed in accordance with variations in the load is necessary.
For high-frequency circuit applications, varicaps and the like that utilize the capacitance between the terminals of a diode are available as capacitors where the capacitance can be electrically controlled. However, due to the capacitance, withstand voltage and the like, varicaps may not be used for power control.
A number of variable capacitors that use a MEMS (Micro Electro Mechanical System) have also been proposed in recent years, but such devices have a premise of using a high-frequency signal.
The capacitance of a capacitor is normally determined by the dielectric constant, the area of the electrodes, and the distance between the electrodes. Accordingly, it is sufficient to control at least one of them. In reality, MEMS proposes a technique that changes the distance between electrodes and/or area of facing electrodes by moving the electrodes.
As another example, Japanese Unexamined Patent Application Publication No. S62-259417 discloses a technology where the capacitance is changed by 70% by applying 50V to a ceramic capacitor to change the dielectric constant. Here, changing the cutoff frequency of a filter circuit or the oscillating frequency of a time-constant oscillator circuit is described as a proposed application.